In order to size and position a femoral knee prosthesis, sizers or calipers are used. The conventional calipers typically follow the referencing method of how the prosthesis grows from size to size. For example, in some knee prosthesis systems, anterior referencing sizing scheme is utilized for the implants where a common anterior/posterior distance from the tip of the anterior flange and the medial/lateral stabilizer pegs (perpendicular from the distal surface of the implant) is maintained for different sizes. The anterior referencing sizing scheme requires that the sizing caliper that sizes the distal femoral implant must measure in the same manner, in which the device will maintain a common distance for drill holes referenced in later steps in the procedure and grow in the opposite direction (i.e. posterior direction).
In other knee prosthesis systems, posterior referencing sizing scheme is utilized. In the posterior referencing sizing scheme, a common anterior/posterior distance from the posterior condyles and the medial/lateral stabilizer pegs is maintained for different sizes. Similar to the anterior referencing systems, the posterior referencing system also imposes an asymmetric requirement on the sizing caliper. The sizing caliper must measure in the manner in which the caliper maintains a common distance for the drill holes referenced in later steps in the procedure and grow in the opposite direction (i.e. anterior direction).
These conventional measuring instruments are dependent on the implant systems that drive them, but create additional problems for the ones they attempt to solve. For the case of an anterior referencing system, the attempt to prevent notching the anterior cortex (a condition where an undercut is created in the anterior cortex resulting from improper posterior positioning of subsequent instruments) will create all bone resections to pull away from the posterior condyles. This can create issues with the flexion/extension gap and create a tighter implant that limits flexion or creating instability in the implant. Additionally, posterior condylar offset can be affected, resulting in conditions where the patient may not obtain as much flexion as desired by the surgeon. Conversely, for a posterior referencing implant, the attempt to maintain flexion/extension gap (and posterior condylar offset) kinematics could create a condition where the surgeon may notch the anterior cortex, thus creating a stress riser and possible fracture conditions. Additionally, mal-position or inaccurate sizing of a posterior referencing system may negatively affect the patella-femoral gap, which may also impact the ability for the replaced knee to achieve full flexion or extension.